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Date: Mon, 4 Mar 2024 21:50:27 +0100
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Subject: Re: [PATCH v3 1/4] mm: swap: Remove CLUSTER_FLAG_HUGE from
 swap_cluster_info:flags
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To: Ryan Roberts <ryan.roberts@arm.com>,
 Andrew Morton <akpm@linux-foundation.org>,
 Matthew Wilcox <willy@infradead.org>, Huang Ying <ying.huang@intel.com>,
 Gao Xiang <xiang@kernel.org>, Yu Zhao <yuzhao@google.com>,
 Yang Shi <shy828301@gmail.com>, Michal Hocko <mhocko@suse.com>,
 Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org
References: <20231025144546.577640-1-ryan.roberts@arm.com>
 <20231025144546.577640-2-ryan.roberts@arm.com>
 <d108bd79-086b-4564-838b-d41afa055137@redhat.com>
 <6541e29b-f25a-48b8-a553-fd8febe85e5a@redhat.com>
 <ee760679-7e3c-4a35-ad53-ca98b598ead5@arm.com>
 <ba9101ae-a618-4afc-9515-a61f25376390@arm.com>
 <2934125a-f2e2-417c-a9f9-3cb1e074a44f@redhat.com>
 <049818ca-e656-44e4-b336-934992c16028@arm.com>
 <d2fbfdd0-ad61-4fe2-a976-4dac7427bfc9@redhat.com>
 <b642c7ff-c452-4066-ac12-dbf05e215cb9@arm.com>
 <949b6c22-d737-4060-9ca1-a69d8e986d90@redhat.com>
 <9ed743a7-0c5d-49d9-b8b2-d58364df1f5f@arm.com>
From: David Hildenbrand <david@redhat.com>
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>>>
>>> This is the existing free_swap_and_cache(). I think _swap_info_get() would break
>>> if this could race with swapoff(), and __swap_entry_free() looks up the cluster
>>> from an array, which would also be freed by swapoff if racing:
>>>
>>> int free_swap_and_cache(swp_entry_t entry)
>>> {
>>>      struct swap_info_struct *p;
>>>      unsigned char count;
>>>
>>>      if (non_swap_entry(entry))
>>>          return 1;
>>>
>>>      p = _swap_info_get(entry);
>>>      if (p) {
>>>          count = __swap_entry_free(p, entry);
>>
>> If count dropped to 0 and
>>
>>>          if (count == SWAP_HAS_CACHE)
>>
>>
>> count is now SWAP_HAS_CACHE, there is in fact no swap entry anymore. We removed
>> it. That one would have to be reclaimed asynchronously.
>>
>> The existing code we would call swap_page_trans_huge_swapped() with the SI it
>> obtained via _swap_info_get().
>>
>> I also don't see what should be left protecting the SI. It's not locked anymore,
>> the swapcounts are at 0. We don't hold the folio lock.
>>
>> try_to_unuse() will stop as soon as si->inuse_pages is at 0. Hm ...
> 
> But, assuming the caller of free_swap_and_cache() acquires the PTL first, I
> think this all works out ok? While free_swap_and_cache() is running,
> try_to_unuse() will wait for the PTL. Or if try_to_unuse() runs first, then
> free_swap_and_cache() will never be called because the swap entry will have been
> removed from the PTE?

But can't try_to_unuse() run, detect !si->inuse_pages and not even 
bother about scanning any further page tables?

But my head hurts from digging through that code.

Let me try again:

__swap_entry_free() might be the last user and result in "count == 
SWAP_HAS_CACHE".

swapoff->try_to_unuse() will stop as soon as soon as si->inuse_pages==0.


So the question is: could someone reclaim the folio and turn 
si->inuse_pages==0, before we completed swap_page_trans_huge_swapped().

Imagine the following: 2 MiB folio in the swapcache. Only 2 subpages are 
still references by swap entries.

Process 1 still references subpage 0 via swap entry.
Process 2 still references subpage 1 via swap entry.

Process 1 quits. Calls free_swap_and_cache().
-> count == SWAP_HAS_CACHE
[then, preempted in the hypervisor etc.]

Process 2 quits. Calls free_swap_and_cache().
-> count == SWAP_HAS_CACHE

Process 2 goes ahead, passes swap_page_trans_huge_swapped(), and calls 
__try_to_reclaim_swap().

__try_to_reclaim_swap()->folio_free_swap()->delete_from_swap_cache()->put_swap_folio()->
free_swap_slot()->swapcache_free_entries()->swap_entry_free()->swap_range_free()->
..
WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries);


What stops swapoff to succeed after process 2 reclaimed the swap cache 
but before process 1 finished its call to swap_page_trans_huge_swapped()?


> 
> That just leaves shmem... I suspected there might be some serialization between
> shmem_unuse() (called from try_to_unuse()) and the shmem free_swap_and_cache()
> callsites, but I can't see it. Hmm...
> 
>>
>> Would performing the overall operation under lock_cluster_or_swap_info help? Not
>> so sure :(
> 
> No - that function relies on being able to access the cluster from the array in
> the swap_info and lock it. And I think that array has the same lifetime as
> swap_map, so same problem. You'd need get_swap_device()/put_swap_device() and a
> bunch of refactoring for the internals not to take the locks, I guess. I think
> its doable, just not sure if neccessary...

Agreed.

-- 
Cheers,

David / dhildenb